Positioning apparatus with relatively moveable members and a linear motor mounted thereon

ABSTRACT

A positioning apparatus comprising first and second relatively moveable members, and a linear motor comprising at least one elongate stator assembly and at least one armature assembly respectively mounted to the first and second moveable members for effecting relative movement of the first and second moveable members, configured so at to permit longitudinal expansion and/or contraction of the at least one elongate stator assembly and/or of the at least one armature assembly relative to its respective member.

This invention relates to a linear motor, in particular to a linearmotor for a positioning apparatus, such as a coordinate measuringmachine (CMM).

A positioning apparatus, such as a CMM, can comprise one or more membersthat are moveable relative to each other for positioning a tool, such asan inspection device, relative to a workpiece/artefact. For example, aCMM traditionally comprises a plurality of moveable members, e.g.linearly moveable members arranged in series. Generally, positioningapparatus are configured to facilitate relative motion of a tool and/orobject in at least two or three mutually orthogonal dimensions, e.g. X,Y and Z. Such positioning apparatus are commonly known as “Cartesian”positioning apparatus (or Cartesian CMM). Typical Cartesian coordinatepositioning apparatus include Bridge, Portal, Cantilever, HorizontalArm, and Gantry type machines.

Whilst the use of linear motors to control the movement of therelatively moveable members of a CMM is known (e.g. such as in the PcMMPlus DCC CMM by Helmel Engineering Products, Inc and the Revolution® CMMby Aims Metrology), CMMs more commonly use conventional (non-linear)motors, e.g. rotary motors which drive belts to effect motion of therelatively moveable members.

The present invention relates to an improved positioning apparatus. Inparticular, the present invention relates to a coordinate positioningapparatus (such as a CMM) comprising a linear motor for controlling themovement of relatively moveable members. The linear motor could, forexample, comprise an ironless core linear motor. The stator and/orarmature of the linear motor could be mounted so at to permit relativelongitudinal expansion and/or contraction of it with respect to themember of the apparatus it is mounted to.

The present invention provides an improvement to the arrangement formounting the stator and/or armature of a linear motor.

According to a first aspect of the invention there is provided apositioning apparatus comprising first and second relatively moveablemembers, and a linear motor comprising at least one elongate statorassembly and at least one armature assembly respectively mounted to thefirst and second moveable members for effecting relative movement of thefirst and second moveable members. The positioning apparatus can beconfigured so at to permit longitudinal expansion and/or contraction ofthe at least one elongate stator assembly and/or of the at least onearmature assembly relative to its respective member.

The arrangement of the present invention can ensure that the statorand/or armature of the linear motor is decoupled from the member onwhich it is mounted in at least one degree of freedom. Decoupling thestator and/or armature of the linear motor in this way has been found toresult in improved metrology of the positioning apparatus. For example,it has been found that relative thermal expansion and/or contraction ofthe stator and/or respective member can adversely affect the metrologyof the positioning apparatus (and the same can be true for the armatureand its respective member). Such relative thermal expansion and/orcontraction could arise, for example, due to the motor heating thearmature and/or stator more quickly than its respective member. Suchrelative thermal expansion and/or contraction could arise, for example,due to ambient temperature changes causing thermal expansion and/orcontraction of the stator and its relative member at different rates(e.g. because they have different thermal inertias, for instance becausethey could be made from different types of material).

As will be understood, the elongate stator assembly can comprise alinear array of magnets. For example, the elongate stator assembly couldcomprise a linear stator assembly. The armature assembly could comprisean elongate armature assembly. The armature assembly could comprise alinear armature assembly, for instance comprising a linear array ofcoils. The coils of the armature could be non-overlapping. For example,the armature could comprise an elongate linear armature.

Optionally, the linear motor is substantially straight. Accordingly, theelongate stator assembly could be substantially straight. The armatureassembly could be substantially straight.

The at least one elongate stator assembly could be mounted to itsrespective member at/toward a first end by a rigid mount assembly. Theat least one elongate stator assembly could be mounted to its respectivemember at/toward its other end via a compliant mount assembly. The atleast one armature assembly could be mounted to its respective memberat/toward a first end by a rigid mount assembly. The at least onearmature assembly could be mounted to its respective member at/towardsits other end via a compliant mount assembly.

A compliant mount assembly can be configured to permit relativeexpansion/contraction of the elongate stator assembly and its respectivemember (to which it is mounted). A compliant mount assembly can beconfigured to permit relative expansion/contraction of the armatureassembly and its respective member (to which it is mounted).

A compliant mount assembly could comprise a spring/flexible member. Inother words, the compliant mount assembly could be configured to changeshape/bend/deform so as to permit relative expansion/contraction of theelongate stator assembly/armature assembly and its respective member (towhich it is mounted). A compliant mount assembly could comprise asliding mount. In other words, the sliding mount could be configured topermit relative sliding of the elongate stator assembly/armatureassembly and its respective member (to which it is mounted), at leastat/toward a first end thereof.

The sliding mount could comprise a lug received within, and moveablealong, an elongate slot. The elongate slot could be provided by theelongate stator assembly and/or armature assembly. Accordingly, the lugcould be provided by the respective member to which the elongate statorassembly and/or armature assembly is mounted.

The elongate stator assembly and/or armature assembly could be biasedtowards its respective member. A spring, e.g. a disc spring couldprovide such bias. Optionally, a magnet provides the bias. Accordingly,the elongate stator assembly and/or armature assembly could bemagnetically biased towards its respective member. The magnets of theelongate stator assembly could provide said bias. Optionally, theelongate stator assembly's and/or armature assembly's mount assembly(e.g. the compliant mount assembly) comprises a magnet so as to providesaid magnetic bias. The sliding mount could comprise a magnet so as toprovide said magnetic bias. Such aforementioned bias (e.g. a bias devicesuch as a spring and/or magnet) could be provided, at least at/towardsthe compliant mount end of the stator and/or armature assembly.

The body of the stator and/or armature could comprise a materialdifferent to that of its respective member. Optionally, the body of thestator and/or armature could comprise the same material as itsrespective member.

There could be provided a plurality of elongate stator assemblies and/ora plurality of armature assemblies. Accordingly, each of the elongatestator assemblies (and/or each of the armature assemblies) could beindependently configured so at to permit longitudinal expansion and/orcontraction of the at least one elongate stator assembly and/or of theat least one armature assembly relative to its respective member. Thatis, the elongate stator assemblies (and/or the armature assemblies)could be mounted to their respective member, independently to eachother.

The elongate stator assembly could comprise a plurality of statormodules. Each stator module could comprise a single, unitary body (e.g.U-shaped body) comprising a plurality of magnets). The plurality ofstator modules could be connected together (end-to-end), e.g. viaplates.

The linear motor could comprise an ironless core linear motor.

The positioning apparatus could be a coordinate positioning apparatus,for example a coordinate measuring machine (CMM), for example aCartesian CMM.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the following drawings, in which:

FIG. 1 is a schematic isometric view of the front of a gantry-type CMMaccording to the present invention;

FIG. 2 is a schematic isometric view of the rear of the CMM of FIG. 1;

FIG. 3 is a schematic isometric view of the cross-beam of the CMM ofFIG. 1;

FIG. 4 is a cross-sectional view of the cross-beam of FIG. 3;

FIGS. 5 and 6 are detail views of the area A identified in FIG. 4;

FIGS. 7a and 7b are detail views of the area A′ identified in FIG. 6;

FIG. 8 is a flow chart illustrating an example method of manufacturingthe cross-beam of FIG. 3;

FIG. 9 shows a bulkhead in isolation;

FIG. 10 is a cross-sectional view showing how the load bearing facets ofthe box structure of the cross-beam of FIG. 3 is riveted to thebulkheads;

FIGS. 11a and 11b show the linear motor arrangement for the CMM'sy-axis, with FIG. 11b being a detail view of the area A″ identified inFIG. 11 a;

FIG. 12a shows a stator assembly of the linear motor of FIG. 11 inisolation;

FIG. 12b shows a module of the stator assembly of FIG. 12a in isolation;

FIGS. 13 and 14 shows the compliant mount assembly of the statorassembly of FIG. 12 in plan and cross-sectional view respectively;

FIG. 15 shows the fixed mount assembly of the stator assembly of FIG. 12in cross-sectional view; and

FIG. 16 shows an isometric view of an armature assembly of the linearmotor of FIG. 3.

FIG. 17 shows a schematic isometric view of the gantry CMM of FIG. 1with a protective housing located over one of the raised rails of they-axis;

FIG. 18 shows the protective housing of FIG. 17 in isolation;

FIG. 19 shows a cut-away view of the protective housing of the cover ofFIGS. 17 and 18;

FIG. 20 shows a partial cross-sectional view of the protective housingof FIGS. 17 to 19;

FIG. 21 shows the energy chain arrangement for the z-axis of the CMM ofFIG. 1;

FIG. 22 shows a side view of the energy chain arrangement of FIG. 21with the quill at a lowered position;

FIG. 23 shows a side view of the energy chain arrangement of FIG. 21with the quill at a raised position; and

FIG. 24 is a schematic isometric view of the rear of the CMM of FIG. 1.

An overview of an embodiment of how the invention can be implementedwill be described below. In this case, the invention is implemented aspart of a CMM 100. FIG. 1 shows a CMM 100 with its protectivehousings/covers (e.g. “main” covers/“hard” covers) removed so that therelevant components of the CMM 100 can be seen.

As shown, a tool, for example an inspection device such as a probe 102for inspecting a workpiece, can be mounted on the CMM 100. In theembodiment shown, the probe 102 is a contact probe, in particular acontact analogue scanning probe, for measuring the workpiece by a stylusof the probe contacting the workpiece. However, as will be understoodthe CMM 100 could carry any sort of inspection device, includingtouch-trigger probes, non-contact (e.g. optical) probes, or another typeof instrument if desired.

In the embodiment shown, the CMM 100 is a gantry-style Cartesian CMM andcomprises a platform 105 on which an artefact to be inspected can beplaced, and a movement system which provides for repeatable and accuratecontrol of the position of the probe 102 relative to the platform 105 inthree orthogonal degrees of freedom X, Y and Z.

In particular, the movement system comprises a cross-beam 106, acarriage 108, and a quill 110. The cross-beam 106 extends between first112 and second 114 raised rail members and is configured to move alongthe rails along a Y axis via a bearing arrangement (in this embodimentan air bearing arrangement). The carriage 108 sits on and is carried bythe cross-beam 106, and is moveable along the cross-beam along an X axisvia a bearing arrangement (in this embodiment an air bearing arrangementwhich is explained in more detail below). The quill 110 is held by thecarriage 108, and is moveable relative to the carriage 108 along a Zaxis via a bearing arrangement (again, in this embodiment via an airbearing arrangement). A pneumatic counterbalance mechanism for the quillis provided for counterbalancing the weight of the quill 110 so as toreduce the work required of the quill's motor. In particular, thepneumatic counterbalance is configured to provide an opposing forcesubstantially equal to the weight of the quill 110 (and the articulatedhead 116 and probe 102) such that substantially zero force is requiredby the quill's motor to keep it at a stationary position. The pneumaticcounterbalance comprises a piston (not shown) within the quill 110. Thepiston is anchored to a tower 194 (in this case a carbon-fibre tube) viaa cable 196. The tower 194 is mounted to the carriage 108 so as to movetherewith.

As will be understood, motors, for example direct drive motors such aslinear motors, can be provided for effecting the relative motion of thevarious members along their axis. Also, position encoders (not shown)can be provided for reporting the position of the cross-beam 106,carriage 108 and/or quill 110.

In the particular example shown, an articulated head 116 is provided onthe lower free end of the quill 110 for carrying the probe 102. In thiscase, the articulated head 116 comprises two orthogonal rotational axes.Accordingly, in addition to the three orthogonal linear degrees offreedom X, Y and Z, the probe 102 can be moved about two orthogonalrotational axes (e.g. A and B axes). A machine configured with such anarticulated head is commonly known as a 5-axis machine.

Articulated heads for tools and inspection devices are well known, andfor example described in WO2007/093789. As will be understood, anarticulated head need not necessarily be provided, and for example theprobe 102 could be mounted to the quill assembly 110 via a fixed headwhich does not provide any rotational degrees of freedom. Optionally,the probe itself can comprise an articulated member so as to facilitaterotation about at least one axis.

As is standard with measuring apparatus, a controller 118 can beprovided which is in communication with the CMM's motors and positionencoders (not shown), the articulated head 116 (if present) and theprobe 102 so as to send and/or receive signals to and/or from them so asto control the motion of the relatively moveable members as well asreceive feedback and measurement data. A computer 127, e.g. a personalcomputer (which can be separate to or integrated with the controller118) can be provided which is in communication with the controller 118.The computer 127 can provide a user friendly interface for an operatorto, for example, program and initiate measurement routines. Suitablecomputers and associated control/programming software is widelyavailable and well known. Furthermore, a joystick 125 or other suitableinput device can be provided which enables an operator to manuallycontrol the motion of the probe 102. Again, such joysticks are wellknown and widely available.

The structure of the cross-beam 106 will be described in more detailwith reference to FIGS. 3 to 10. As shown, in this embodiment thecross-beam 106 comprises a box beam. The box beam 106 has a modularconstruction, and in particular in this embodiment comprises threeelongate corner members 120, 122, 124, and three pieces of sheetmaterial 126, 128, 130, each forming a load bearing facet (in otherwords, each forming a planar load bearing member) of the box beam, andeach extending between a pair of the three elongate corner members. Inthe embodiment described, the elongate corner members 120, 122, 124 areextruded, i.e. formed via extrusion. In the embodiment described, thethickness of the three pieces of sheet material 126, 128, 130 is notmore than 3 mm. They are made from a metallic material, in particular inthis embodiment aluminium, although as will be understood other metallicmaterials such as stainless steel could be used, or non-metallicmaterials such as carbon fibre or ceramic could be used. In order toreduce bulk and weight, it can be preferred that the load bearing facets(i.e. the planar load bearing members) of the box beam are formed fromsheet material no thicker than 5 mm (above which they would morenormally be described as being “plates” rather than “sheets”).

If desired the three elongate corner members 120, 122, 124 could be madeto be substantially identical. This could help to ensure that the threeelongate corner members have substantially the same thermal inertia(e.g. same thermal response characteristics) such that they respond totemperature changes in a common way. This can help to avoid deformation(e.g. twisting or bending) of the box beam 106. For the same reasons,the three pieces of sheet material 126, 128, 130 could also be made soas to be substantially identical. However, as will be understood, thecorner members (and/or pieces of sheet material) could be designed tohave the same thermal inertia so as to achieve the same effect, even ifthey are not substantially identical, e.g. even if they do not have thesame shape or cross-sectional form.

In the described embodiment, the three elongate corner members 120, 122,124 and the three pieces of sheet material 126, 128, 130 are formed fromthe same material type (e.g. aluminium).

In the described embodiment, the first elongate corner member 120provides first 132 and second 134 bearing surfaces against which airbearings can bear. In the described embodiment, the carriage 108comprises first and second air bearing assemblies which each comprisefirst 140 and second 142 air bearing pads connected to each other and tomain body 109 of the carriage 108 via a mounting bracket 139 (omittedfrom FIGS. 4 to 6). The first and second air bearing assemblies straddlethe first elongate corner member 120, such that the first air bearingpads 140 bear against the first bearing surface 132 and the second airbearing pads 142 bear against the second bearing surface 134.

In its assembled state, the box beam 106 and carriage 108 are pre-loadedagainst each other. Such pre-load could be provided by gravity and/or byspring loading. For example, air bearings pads 140, 142, 143 (see FIG.4) could be rigidly mounted to the carriage 108 (not shown in FIG. 4)and air bearing pad 145 could be spring mounted to the carriage 108 toprovide the pre-load.

As schematically illustrated in FIG. 5, the pre-load causes the first140 and second 142 bearing pads to exert a force on the box beam 106,respectively illustrated by first and second vectors F₁, F₂. Asillustrated, the apparatus is configured such that the forces F₁, F₂intersect at the same point at which the planes of the first 126 andsecond 128 pieces of sheet material intersect. This ensures that theforces transferred into the first elongate corner member 120 can beresolved (and hence the forces can be transferred) directly into/alongthe (e.g shear) plane of the first 126 and second 128 pieces of sheetmaterial. Accordingly, the pre-load force, is carried directly along theplane of the sheet material. This helps to avoid buckling of the sheetmaterial and can mean that thinner (and therefore lighter) sheets can beused to support a given pre-load compared to a configuration in whichthe pre-load forces cannot be carried directly along the plane of thesheet material.

In the embodiment shown, first 140 and second 142 bearing pads arearranged to straddle the first elongate corner member 120. It is knownthat the forces F₁, F₂ will be transferred perpendicularly into thefirst 132 and second 134 bearing surfaces of the first elongate cornermember 120. It therefore follows that the forces F₁, F₂ from the first140 and second 142 bearing pads will intersect at a predictable point(point 150 shown in FIGS. 6 and 7). This point is predictable along thelength of the first elongate corner member 120, and so could bedescribed as being a predictable intersection line. In other words, inthis embodiment the forces imparted into the first elongate cornermember 120 by the first 136 air bearing assembly at each of the pointsof cooperation between the two is directed so as to intersect apredetermined elongate target line that extends parallel to the firstelongate corner member 120. Since the point of intersection 150 (andhence the elongate target line) is known and is predictable, it ispossible to configure the box beam 106 such that the planes 152, 154 ofthe first 126 and second 128 pieces of sheet material also intersect atsubstantially the same point (along the same line).

Moreover, as illustrated by FIGS. 6 and 7, in order to ensure that thepre-load forces are primarily carried in/along the (e.g. shear) plane ofthe first 126 and second 128 pieces of sheet material, it is possible toconfigure the box beam 106 such that the point of intersection 150 (andi.e. the elongate target line) falls within the vicinity of a notionalelongate volume (a cross-section of which is highlighted by the diamondshape 170 shown in FIGS. 7a and 7b ) defined by the intersection of afirst pair of planes 160 containing the front and back surfaces of thematerial of the first piece of sheet material 126 (which defines a firstload bearing facet/planar load bearing member) with a second pair ofplanes 162 containing the front and back surfaces of the material of thesecond piece of sheet material 128 (defining a second load bearingfacet/planar load bearing member). In this embodiment, this isfacilitated by making the bearing surfaces (e.g. 132, 134) of theelongate bearing tracks (e.g. 120) sit substantially proud relative tothe adjacent pieces of sheet material (e.g. 126, 128). In this case, thestep S between the surface of the adjacent pieces of sheet material andthe bearing surfaces is approximately 18 mm. Also, as indicated in FIG.5, the extruded bearing tracks 120 are substantially hollow, butcomprise a plurality of reinforcing webs 121, 123. As shown, there isone web on each side of the corner which (i.e. web 123) extendsperpendicular to the bearing surfaces 132, 134 and is located centrallywith respect to the bearing pads 140, 142, such that the pre-load iscarried directly through it.

Whilst it can be preferred that the point of intersection 150 fallsinside said notional elongate volume 170, it can be sufficient for saidpoint of intersection 150 to be in the vicinity of said notionalelongate volume 170. For example, as illustrated in FIG. 7b , it can besufficient for said point of intersection to be within a greaternotional volume 172 which is centred on, but having up to 100%cross-sectional area than that of the notional elongate volume definedby the intersection of a first pair of planes 160 containing the frontand back surfaces of the material of the first piece of sheet material126 with a second pair of planes 162 containing the front and backsurfaces of the material of the second piece of sheet material 128.Rather than being measured proportionally, the greater notional volume172 could be determined absolutely, e.g. as illustrated in FIG. 7b , thegreater notional volume which is centred on the notional elongatevolume, could have a cross-sectional extent that is greater than that ofthe notional elongate volume by not more than 5 mm on all sides. Such aconfiguration can help to ensure that the pre-load forces are primarilycarried in/along the (e.g. shear) planes of the first 126 and second 128pieces of sheet material.

The same bearing arrangement is provided between the bearing assemblieson the carriage 108 and the second elongate corner member 122 asschematically illustrated in FIG. 4, such that the pre-load forcesimparted into the second elongate corner member 122 is primarily carriedin/along the (e.g. shear) planes of the second 128 and third 130 piecesof sheet material.

Since the pre-load forces are primarily carried in/along the (e.g.shear) planes of the first 126, second 128 and third 130 pieces of sheetmaterial of the box beam 106, the inventors have found that othersupporting structures like bulkheads are not necessary for supportingthe pre-load forces. However, as shown in FIG. 3, the box beam 106 ofthe present embodiment does have a plurality of bulkheads 180 (shown inisolation in FIG. 9). Providing bulkheads can help manufacture of thebeam. The bulkheads can also aid assembly of the different pieces of thebox beam by holding them in place during assembly. Also, if the elongatecorner members need to be machined to improve their bearing surfaces,and if this machining is done after assembly of the box beam 106, thenthe bulkheads can help to provide support during such machining. FIG. 8illustrates an example process 10 for manufacturing the box beam 106. Asillustrated, after manufacture of the different parts of the box beam106 (e.g. after extrusion of the first to third elongate corner membersat step 12 and cutting of the first to third pieces of sheet materialand the bulkheads at step 14) they are assembled into the box beam atstep 16. (As will be understood, the manufacturing steps 12 and 14 couldbe performed by different parties at different stages to the assemblystep 16). In the described embodiment, the assembly step 16 involvesjoining the first 126, second 128 and third 130 pieces of sheet materialto the bulkheads 180 and attaching the first 120, second 122 and third124 elongate corner members to the first 126, second 128 and third 130pieces of sheet material.

As shown, the bulkheads 180 are, in the described embodiment, pop/blindriveted “end-on” to the first 126, second 128 and third 130 pieces ofsheet material (e.g. as opposed to a folded flap on the bulkheads). Thisensures that loads which are directed orthogonally into the first 126,second 128 and third 130 pieces of sheet material are primarily carriedin/along the (e.g. shear) plane of the bulkhead 180 enabling them to bemade from thinner sheets of material (thereby saving weight). Such anarrangement is possible by the provision of recesses 182 (see FIG. 9) inthe edges of the bulkheads which have a narrowed/restricted neck 184,through which the pop/blind rivets 188 can be accepted. When the rivetis expanded, it can grip against the sides of the recess 182 (e.g.against an inside shelf 186 at the end of the neck 184) thereby securingthe bulkhead to the sheet of material (e.g. the first piece of sheetmaterial 126 as shown in FIG. 10) which provides the load bearingfacet/planar load bearing member of the box beam 106.

In the described embodiment, the varies pieces of the beam 106 are thenglued together using adhesive. For example, the first 120, second 122and third 124 elongate corner members are glued to the first 126, second128 and third 130 pieces of sheet material (e.g. via an appropriateadhesive, such as a single part, heat cured, epoxy, for examplePERMABOND® ES569 available from Permabond Engineering AdhesivesLimited). Also, the bulkheads 180 can be glued to the first 126, second128 and third 130 pieces of sheet material (e.g. using the sameadhesive).

Once assembled, the box beam 106 is then loaded into a machine tool (notshown) at step 18 (see FIG. 8). In the embodiment described, this isdone via the end bulkheads 180 which have mounting features in the forma hole 190 which a corresponding mating member on the machine tool canengage. In view of this, the end bulkheads can be thicker than the innerbulkheads in order to withstand the mounting forces. For example, theend bulkheads could be 6 mm thick whereas the inner bulkheads could be 3mm thick since the inner bulkheads.

Once loaded into the machine tool, the first 122 and second 122 elongatecorner members are machined at step 20 to improve the finish of the airbearing surfaces (e.g. 132, 134), e.g. to make them flatter/smoother andoptionally to improve how parallel they are to each other.

In the embodiment described a direct drive motor 200, in particular alinear motor 200, is used to drive the cross-beam 106 along the y-axis.A linear motor can be advantageous in that it can help to facilitate aservo system with high servo stiffness. The arrangement of the linearmotor 200 on the CMM 100 is shown in FIGS. 11a and 11b , and will bedescribed in more detail in connection with FIGS. 1 to 16. As shown, thelinear motor 200 comprises a stator 202 and an armature 204. Thearmature 204 is mounted to the cross-beam 106 (which in this embodimentis formed from aluminium), and the stator 202 is mounted to the secondraised rail member 114 (also formed from aluminium). As will beunderstood, the armature 204 comprises a plurality of coils 206 mountedto a body 205 (e.g. as shown in FIG. 16) and the stator 202 comprises aplurality of magnets 208 mounted along its length on opposing innersides of a U-shaped body 207 (e.g. as shown in FIGS. 12 to 14), so as todefine a channel 209 in which the armature can be received. In theembodiment described the U-shaped body 207 comprises a steel material,which is particularly appropriate for a linear motor stator body (forcontaining the magnetic field of the stator's magnets 208 and improvingthe magnetic flux density). Also, in the embodiment described, the body205 of the armature is made from a non-ferrous material such asaluminium. As will be understood by those familiar with linear motors,current can be passed through the armature's coils 206 in a controlledmanner so as to cause the armature 204 (and hence the cross-beam 106 towhich it is fixed) to be pushed along the stator 202 (and hence alongthe first 112 and second 114 raised rail members). In the embodimentdescribed, the linear motor is an ironless core linear motor. Thisreduces forces between the armature and stator in directions other thanin the direction of motion, thereby reducing the stiffness requirementsof their respective mounts, and thereby reducing forces on the metrologyloop (which could vary along the axis if the armature and stator are notperfectly aligned). As shown, the coils of the armature are notoverlapping.

In this embodiment, air bearings facilitate low-friction motion betweenthe cross-beam 106 and the first 112 and second 114 raised rail members.In particular, at a first end of the cross-beam 106 there is provided afirst air bearing arrangement comprising an air bearing pad 250 whichbears against the first raised rail member 112. At the opposing, secondend, of the cross-beam 106 there is provided a second air bearingarrangement comprising a plurality of air bearing pads 252 which bearagainst different facets of the second raised rail member 114. As willbe understood, additional air bearing pads to those shown may beprovided, e.g. so as to provide a pre-load between the beam 106 and thefirst 112 and second 114 raised rail members. As will be understood,other types of bearing, including mechanical bearings, can be used aswell as or instead of the air bearings.

In the embodiment described, the stator 202 comprises a plurality ofstator modules 220 (which in this embodiment are identical, althoughthis need not necessarily be the case) which are connected to each othervia connector members 222 (in this case plates 222 which are bonded toadjacent stator modules) so as to provide two stator assemblies. Inparticular, a first stator assembly comprises first 220 a, second 220 band third 220 c stator modules connected in series via plates 222, and asecond stator assembly comprises fourth 220 d, fifth 220 e and sixth 220f stator modules connected in series via plates. FIG. 12a shows a statorassembly in isolation comprising a plurality of stator modules (e.g. 220a, 220 b, 220 c) connected via plates 222. As will be understood, astator assembly can essentially be considered to be equivalent to onestator module, and so the explanations below in connection with thestator assembly is equally applicable to a stator assembly comprising asingle stator module (shown in FIG. 12b in isolation) and vice versa. Inother words, a stator assembly could comprise just a single statormodule (e.g. having just a single unitary U-shaped body, rather thanseparate bodies joined together by plates 222)

In the embodiment described, the armature 204 also comprises a pluralityof armature assemblies 224 (which in this embodiment are identical,although this need not necessarily be the case) which are each connectedto a bracket 300. For simplicity, FIG. 16 shows only one armatureassembly 224. As will be understood, even though in the describedembodiment there is provided an armature 204 comprising a plurality ofarmature assemblies 224, this need not necessarily be the case, and thearmature could comprise just one armature assembly. (Also, in thedescribed embodiment, each armature assembly 224 comprises just a singlearmature module, but as with the stator assembly of the describedembodiment, an armature assembly 224 could comprise a plurality ofarmature modules connected together, e.g. via plates. As per thecomposite stator assembly, such a composite armature assembly could befixed to the bracket toward a first end via the rigid mounting of one ofthe armature modules, and fixed to the bracket toward a second end viathe flexible mounting of one of the other armature modules).

Such a modular arrangement of the stator and/or armature can aidmanufacture of the CMM 100.

As described in more detail below, each stator assembly and eacharmature assembly is mounted to its respective member in a way whichpermits longitudinal expansion and/or contraction relative to itsrespective member. With regard to the stator assemblies (e.g. the firststator assembly comprising the first 220 a, second 220 b and third 220 cstator modules), this is achieved in the particular embodiment describedby providing the stator assembly with a fixed mounting assembly 260 atone end and a compliant mounting assembly 270 at its other end. Withreference to FIG. 15, the fixed mounting assembly 260 is illustrated. Asshown, the fixed mounting assembly 260 comprises a spacer member 262which is rigidly secured to the second raised rail member 114 (e.g. viabonding and/or screwing) and a screw 264 which extends through ahole/slot 266 provided at a first end of the stator 202/stator module220 (e.g. see FIG. 12). The screw 264 is received in a threaded bore inthe spacer member 262 and is tightened so that the screw's 264 headengages the body 207 of the stator 202/stator module 220 so as torigidly clamp the stator 202/stator module 220 to the spacer member 262and therefore to the second raised rail member 114.

With reference to FIGS. 13 and 14, the compliant mounting assembly 270comprises an elongate slot 268 (see also FIG. 12) formed in the body 207at the first end of the stator 202/stator module 220 (at the endopposite to the hole/slot 266) and a sliding mount. The sliding mountcomprises a spacer member 276 a post member 272 (which extends into theslot to control the transverse location of the stator assembly/statormodule 220) and a magnet 274 which is configured to attract and hold thesteel body 207 of the stator assembly/stator module 220 to the spacermember 276 (and therefore to the second raised rail member 114). In theembodiment described the magnet 274 is ring-shaped and extends aroundthe post member 272. The elongate slot 268 and post member 272 areconfigured so that the stator 202/stator module 220 and post member 272are free to slide relative to each other along the length of theelongate stator assembly/stator module 220 (i.e. in the direction ofarrow A in FIG. 13). By way of such relative sliding, relative expansionand/or contraction of the stator assembly/stator module 220 and themember it is mounted on (in this case the second raised rail member 114)is facilitated. As will be understood, such relative expansion and/orcontraction could be as a result of heat from the motor and/or due todifferences in coefficients of thermal expansion of the parts which meanthat they expand/contract at different rates with changes in ambienttemperature.

As is also shown in FIG. 14, a screw 278 can be screwed into a threadedbore in the post 272. However, unlike the screw 264 of the fixedmounting assembly 260, the head of the screw 278 of the compliantmounting assembly 270 does not engage the body 207 of the stator202/stator module 220 and so does not act to clamp the stator 202/statormodule 220 to the spacer member 276 and therefore to the second raisedrail member 114. Rather, there is a small gap between the head of thescrew 272 and the body 207. Accordingly, the screw 272 merely acts as asafety mechanism to prevent the stator 202/stator module 220 from beingpulled off the second raised rail member 114.

Each of the first and second stator assemblies can be mounted in thisway, with a gap between them to facilitate their expansion. Also, aswill be understood, rather than connected stator modules into statorassemblies, each stator module could be connected individually, forexample in the way described above, with gaps between each of them tofacilitate their expansion. Alternatively, there could be provided justone monolithic stator module (again mounted in the manner describedabove via fixed and compliant mounting assemblies). This is also thecase for the armature as described in more detail below.

As will be understood, such expansion/contraction can be facilitated inother ways. For example, with reference in particular to FIG. 16, anarmature assembly 224 of the armature 204 comprises a fixed mountingassembly 290 at one end and a compliant mounting assembly 292 at anotherend. The fixed mounting assembly 290 comprises a screw 291 which extendsthrough a hole in the body 205 of the armature module 224 and engages athreaded bore in a bracket 300 (which is in turn rigidly mounted to thecross-beam 106) so as to rigidly clamp the body 205 of the armatureassembly 224 to a bracket 300. At the other end, the compliant mountingassembly 292 comprises a flexure arm 294. A first end of the flexure armis screwed rigidly to the bracket 300 via (in this case) two screws 293,and at the second end is attached to the body 205 of the armature module224. The flexure arm 294 is configured to flex in the longitudinaldirection of the armature module 224 (i.e. in the direction of arrow B)so as to facilitate relative expansion and/or contraction of the bracket300 and the armature module 224, but is relatively stiff in directionsperpendicular thereto (i.e. in directions perpendicular to arrow B).

Such an arrangement could be used in place of the sliding mount of thestator module 220/stator 202, and vice versa.

The arrangements described help to accommodate longitudinal expansionand/or contraction of the armature assembly and/or stator assemblyrelative to its respective member, whilst maintaining the servostiffness of the apparatus.

In the embodiment described, both the stator assemblies and the armatureassemblies are mounted to their respective members in a way whichpermits longitudinal expansion and/or contraction relative to itsrespective member. However, as will be understood, it is possible forjust the stator assemblies or just the armature assemblies to be mountedin such a way to permit longitudinal expansion and/or contractionrelative to its respective member.

The linear motor arrangement is described above in connection with theCMM's y-axis. As will be understood, the same or a similar arrangementcan be used for effecting motion in the x and/or z axes. Likewise,similar bearing arrangements (e.g. air bearings) can be used for the xand/or z axes.

As will be understood, it is common for CMMs to be provided with one ormore protective housings (covers) to protect various parts of the CMMfrom external contamination and objects. Turning now to FIGS. 17 and 18,there is shown an example of such a type of protective housing (cover)400 configured to protect the linear motor 200 of the CMM's y-axis andalso the above mentioned second air bearing arrangement (comprising theair bearing pads 252, and the respective bearing surfaces on the secondraised rail 114). This protective housing (cover) 400 will be describedin more detail in connection with FIGS. 17 to 20.

The protective housing 400 together with the structure of the CMM 100,in particular the structure of the second raised rail 114 define aninternal volume 402 within which the linear motor 200 and the airbearing pads 252 (and their respective bearing surfaces) of the secondair bearing arrangement are located and protected from contamination andobjects located in the external operating environment 404.

The protective housing 400 comprises first 410 and second 412 endplates, and front 414 and back plates 416 (which in this case are foldedto provide multiple facets as shown in FIG. 19, and is configured to bereceived over and capture the second raised rail 114. The first 410 andsecond 412 end plates are secured to the second raised rail 114 byfasteners (e.g. mechanical fasteners such as screws) to hold it inplace. An elongate opening 401 in the protective housing 400 is providedsuch that the cross-beam 106 can extend into the protective housing andsuch that its bearing pads can cooperate with the second raised rail 114to facilitate guided relative motion with the second raised rail. Theprotective housing 400 further comprises a retractable dust cover in theform of first 420 and second 422 bellows. A bellows frame 424 isprovided for attaching the bellows to the cross-beam 106 such that theyexpand and contract with movement of the beam 106. Upper 430 and lower432 bellows tracks (in the form of channels) are provided, in which theupper and lower sides of the bellows 420, 422 are received, such thatthey are guided as they expand and contract with movement of the beam106.

The first 420 and second 422 bellows expand and collapse/fold withmovement of the cross-beam 106 along the y-axis. In particular, thecross-beam 106 is connected to the frame 424 which slides with thecross-beam 106 so as to push and pull the first 420 and second 422bellows as the cross-beam 106 moves back and forth along the y-axis. Asshown in more detail in FIGS. 19 and 20, the first 420 and second 422bellows sit within and are guided by the first 430 and second 432bellows tracks. In particular, each of the upper 430 and lower 432bellows tracks comprise a channel 434 within which the upper and lowersides/edges of the first 430 and second 432 bellows sit and can slide.

As shown in FIGS. 19 and 20, each channel 434 comprises a contaminationtrap 436. As most clearly shown in FIG. 20, the contamination trap 436comprises a groove 438 which runs along the length of the channel 434 inwhich dirt can collect away from the bellows 422. Also, an elongatemagnetic strip 440 can be located within the groove 438, this canattract and hold ferromagnetic contamination/dirt trying to enter theinternal volume 402 defined by the protective housing 400.

As will be understood, the protective housing 400 does not provide ahermetic seal between the internal volume 402 defined by the protectivehousing 400 and the CMM's external operating environment 404.Accordingly, there will be some flow of air between the internal volume402 and the CMM's external operating environment 404. In particular, dueto the movement of the first 420 and second 422 bellows along thechannels 434, there can be “leakage” between the internal volume 402 andthe CMM's external operating environment 404, for instance around thesides of the bellows 420, 422 as illustrated by dashed arrow A in FIG.20. Dirt and contamination can be entrained in such a flow of air. Ourinventors have found that providing a trap, such as a groove 438, canhelp to reduce the amount of such entrained dirt and contaminationentering the internal volume 402. This can be beneficial in maintainingthe performance, reliability and/or lifespan of the CMM 100, such as theair bearings and motors located in the internal volume 402. Inparticular, providing a magnet 440 in the groove can help to attract,remove and retain ferromagnetic contamination or dirt present in the airflow A. This has been found to be particularly useful in embodiments inwhich the motor comprises a linear motor 200 (which typically compriseplurality of strong, exposed magnets). Such a ferromagnetic trap 436helps to reduce the amount of ferromagnetic contamination reaching themotor linear 200 which would affect the performance and lifespan of thelinear motor 200.

As will be understood, in other embodiments a plurality of (e.g.non-elongate) magnets could be placed in the groove 438, rather than oneelongate strip. Furthermore, the magnet(s) need not be located in agroove. For example, one or more magnets could be located adjacent thechannel 434 (e.g. on any of the surfaces identified by reference numeral439) and would attract and retain at least some of the ferromagneticmaterial entrained in the air flow along A. However, the provision of agroove can help to trap any contamination and dirt, and also helps tokeep such contamination and dirt away from other parts of the CMM,including the first 420 and second bellows 422 (the sliding of whichwould otherwise be affected by the collection of contamination and dirtin the channels 434).

The elongate magnetic strip 440 could be removable. For example, itcould just rest in the groove 438 and/or be held by releasable means,such as a releasable (e.g. mechanical) fastener and could be accessiblefor removal via end caps 442 provided on the end plates 410. Whenopened/removed, such end caps 424 can help to facilitate cleaning and/orreplacement of the elongate magnetic strip 440 (by enabling them to beslid out of the groove), and/or cleaning of the groove 438.

This concept of providing a contamination trap is described above inconnection with the CMM's y-axis. As will be understood, the same or asimilar arrangement can be used for the x and/or z axes.

As is normal on a positioning apparatus such as CMM 100, an energyconduit (or “energy chain”) exists between the moveable members of theapparatus which comprises the necessary wires and pipes such thatelectrical power, signals and/or fluid (such as air for air bearings),can be delivered to and/or from the moveable member (and/or downstreammembers, instruments and the like, such as articulated probe heads andprobes).

With particular reference to FIGS. 21 to 23, in the present embodiment,two energy conduits (first 502 and second 504 energy conduits) areprovided between the quill 110 and the carriage 108 which each compriseone or more electrical wires for providing power and communications toand/or from the quill 110, the articulated probe head 116, and the probe102. The first 502 and second 504 energy conduits can also comprise oneor more pipes for supplying air to the quill's air bearings (not shown).In the embodiment described, each of the first 502 and second 504 energychains comprise a support track which flexes with relative movement ofthe quill 110 and carriage 108. The support tracks are configured tokeep the wires and pipes associated with it tidy and to control how theyflex with the relative movement of the quill 110 and carriage 108. Afirst end of each support track of the first 502 and second 504 energychains is connected to the carriage 108 (in this case to the carriage'scounterbalance tower 194, via bracket 195), and a second end of eachsupport track of the first 502 and second 504 energy chains is connectedto the quill 110 (in this case via a bracket 198).

Providing two energy chains between the relatively moveable members(e.g. between the quill 110 and the carriage 108) means that they can beconfigured such that the load they each impart on the relativelymoveable members varies inversely to each other. For example, ourinventors found that providing just a single energy chain (e.g. firstenergy chain 502) meant that the load imparted on the quill 110 varieddepending on the position of the quill 110 relative to the carriage 108.This is because the energy chain itself imparts a load on the quill 110and carriage 108. For example, in the embodiment described the loadcaused by the weight of the first energy chain 502 shifts from beingpredominately carried by the carriage 108 when the quill 110 is at avertically low position (see FIGS. 21 and 22) to being predominatelycarried by the quill 110 when the quill 110 is at a vertically highposition (see FIG. 23). Such varying load can have an adverse effect onthe metrology of the CMM 100. In particular, our inventors found thatthe quill's motor had to work harder at increased heights of the quill110. In particular, because the motor of this embodiment is a directdrive motor (and in particular a linear motor), it was found that asignificantly varying amount of heat was produced by the motor dependenton the position of the quill 110. As will be understood, the structureof the apparatus can change depending on its temperature and therefore avarying heat source such as the motor can lead to poorer than desiredmetrological performance.

Our inventors found that this effect can be reduced, and even avoided,by providing a compensatory member which is configured to apply a loadthat varies dependent on the relative position of the quill 110 and thecarriage 108, so as to at least partially counteract the change in loadapplied by the first energy conduit 502 (that is dependent on therelative position of the quill 110 and the carriage 108). In theembodiment described, the compensatory member comprises the secondenergy conduit 504 which is connected to the quill 110 and carriage 108in a manner such that the loads they impart on the quill 110 andcarriage 108 vary substantially equally and oppositely. Accordingly, thefirst 502 and second 504 energy conduits could be described as being“balanced”. In the embodiment described, this is achieved by ensuringthat the first 502 and second 504 energy conduits are substantiallyidentical, at least between the members they are connected. For example,the articulated support tracks of the first 502 and second 504 energyconduits are substantially identical in configuration, and the mass ofthe wires and/or pipes are evenly split between the first 502 and second504 energy conduits. As will be understood, benefit can still beobtained even if the load imparted by the compensatory member does notvary substantially equally and oppositely, but it can be preferred thatthe load it imparts does vary substantially equally and oppositely.

As will be understood, other arrangements are possible. For example,rather than substantially equally sharing the wires and pipes betweenthe first 502 and second 504 energy conduits, they could be shared in asubstantially non-equal way. Furthermore, it might be that the secondenergy conduit is a “dummy” energy conduit in that it does notcarry/guide any wires or pipes. Accordingly, the support track of thedummy second energy conduit might be provided merely as a compensatorymember. In this case the support track of the dummy second energyconduit could be configured differently to the support track of thefirst energy conduit such that the load the support track of the dummysecond energy conduit imparts on the members is substantially equal andopposite to that of the first energy conduit (which comprises the trackand the wires and pipes). For example, the mass of the support track ofthe dummy second energy conduit 504 can be greater than that of thesupport track of the first energy conduit 502 to compensate for the massof (and resistance provided by) the wires and pipes of the first energychain 502.

In the embodiment described, the support track of each of the first 502and second 504 energy conduits comprises a chain-like arrangement ofpivotally connected links, but this need not necessarily be the case.For example, the support tracks of the first 502 and second 504 energyconduits could comprise a continuous ribbon-like band of material whichbends with the relative movement of the quill 110 and carriage 108.Optionally, no support tracks are provided and the wires and pipes couldfor example be tied together to keep them tidy. In this case, inaccordance with this embodiment of the invention the wires and pipescould be split into first and second bunches and tied together toprovide the first 502 and second 504 energy chains. Accordingly, in thiscase the second bunch could be considered to be the compensatory member,for example.

The concept of having a compensatory member which is configured to applya load that varies dependent on the relative position of the moveablemembers of the CMM so as to at least so as to at least partiallycounteract the change in load applied by an energy conduit has beendescribed above in connection with the quill 110 and carriage 108. Thisis because the effect of the varying load is most pronounced due to theshift in weight carried between the quill 110 and carriage 108 due tothe relative vertical motion. However, the concept of having such acompensatory member has also been found to be beneficial for the otheraxes of the CMM too, which provide for horizontal relative motion (andso are not subject to varying weight loads in the direction of motion),since the back-driving force applied by an energy conduit to arelatively moveable member can vary depending on the position of themoveable member along the axis. For example, such an arrangement of twosubstantially balanced energy conduits between horizontally moveablemembers can be seen in FIG. 24 where first 602 and second 604 energyconduits are provided between the beam 106 and the carriage 108. As withthe first 502 and second 504 energy conduits between the carriage 108and quill 110, the first 602 and second 604 energy conduits between thebeam 106 and carriage 108 comprise one or more electrical wires and oneor more pipes. Also, as with the first 502 and second 504 energyconduits between the carriage 108 and quill 110, the first 602 andsecond 604 energy conduits between the beam 106 and carriage 108 areconfigured such that the load they impart on their members (e.g. thecarriage 108) varies substantially equally and oppositely with therelative movement of the carriage 108 along the beam 106. However,unlike the first 502 and second 504 energy conduits between the carriage108 and quill 110, the first 602 and second 604 energy conduits comprisecontinuous ribbon-like bands of material which bend with the relativemovement of the carriage 108 and beam 106 so as to guide the wires andpipes (rather than comprising a chain-like arrangement of pivotallyconnected links).

Providing a compensatory member can help to reduce or even avoid anychange in the resultant load caused by the back-driving force. This isparticularly advantageous where a direct drive motor (such as a linearmotor) is used to effect the relative movement due to the abovedescribed heat dissipation issues which direct drive motors (e.g. linearmotors) are particularly sensitive to. In particular, ensuring that thecompensatory member substantially balances the force applied by thefirst energy chain (e.g. such that the resultant load applied to themoveable member by the energy chain and compensatory member is not morethan 5 Newtons (N), and optionally not more than 4N, for example notmore than 3N, for instance not more than 2N or even not more than 1Nalong at least 75%, optionally along at least 90% of its moveable extentalong the axis) can ensure that heat dissipated by the motor is notexcessive. Furthermore, providing a compensatory member which provides aforce to the moveable member which varies inversely to that provided bythe first energy chain such that the change in resultant load applied tothe moveable member by the energy chain and compensatory member is notgreater than 3N, optionally not more than 2N, and for example not morethan 1N along at least 75%, optionally along at least 90%, of itsmoveable extent can ensure that variations in heat dissipated by themotor along the axis is kept within a reasonable level.

In the embodiments described, the bearing assembly comprises an airbearing. However, as will be understood, the invention is alsoapplicable to non-air bearing assemblies. For example, mechanicalbearings, such as ball race bearings, could be used.

As will be understood, the invention and design principles thereof isalso applicable to other parts of the CMM 100 (e.g. to the quill 110),and also to other types of CMM, including bridge, column, horizontal armand cantilevered CMMs (as a non-exhaustive list). The invention is alsonot limited to CMMs, but is applicable to other positioning apparatusincluding machine tools.

The invention claimed is:
 1. A positioning apparatus comprising: firstand second relatively moveable members; and a linear motor comprising atleast one elongate stator assembly and at least one armature assemblyrespectively mounted to the first and second moveable members foreffecting relative movement of the first and second moveable members,and configured so as to permit longitudinal expansion and/or contractionof: the at least one elongate stator assembly relative to andindependent of the first moveable member to which it is mounted, and/orthe at least one armature assembly relative to and independent of thesecond moveable member to which it is mounted.
 2. A positioningapparatus as claimed in claim 1, wherein the at least one elongatestator assembly and/or the at least one armature assembly is mounted toits respective member at least at a first point by a rigid mountassembly, and at least at a second point by a compliant mount assembly.3. A positioning apparatus as claimed in claim 2, wherein the compliantmount assembly comprises a flexible member.
 4. A positioning apparatusas claimed in claim 2, wherein the compliant mount assembly comprises asliding mount.
 5. A positioning apparatus as claimed in claim 4, whereinthe sliding mount comprises a lug received within, and moveable along,an elongate slot.
 6. A positioning apparatus as claimed in claim 2,wherein the elongate stator assembly and/or armature assembly ismagnetically biased towards its respective member.
 7. A positioningapparatus as claimed in claim 6, wherein the sliding mount comprises amagnet so as to provide the magnetic bias.
 8. A positioning apparatus asclaimed in claim 2, wherein the first point is located toward a firstend of the elongate stator assembly/armature assembly, and in which thesecond point is located toward its other end.
 9. A positioning apparatusas claimed in claim 1, wherein the body of the stator and/or armaturecomprises a material different to that of its respective member.
 10. Apositioning apparatus as claimed in claim 1, wherein the body of thestator and/or armature comprises the same material as its respectivemember.
 11. A positioning apparatus as claimed in claim 1, wherein thelinear motor comprises an ironless core linear motor.
 12. A positioningapparatus as claimed in claim 1, wherein the stator assembly comprises asingle stator module.
 13. A positioning apparatus as claimed in claim 1,wherein the stator assembly comprises a plurality of stator modulesconnected together.
 14. A positioning apparatus as claimed in claim 1,wherein the positioning apparatus is a coordinate measuring machine. 15.A positioning apparatus as claimed in claim 1, wherein the first andsecond relatively moveable members are configured to move an inspectiondevice relative to an artefact.
 16. A positioning apparatus as claimedin claim 1, wherein the positioning apparatus is configured tofacilitate relative motion of a tool and/or object in three mutuallyorthogonal dimensions.
 17. A positioning apparatus as claimed in claim1, wherein the positioning apparatus is a Cartesian coordinatepositioning apparatus.
 18. A positioning apparatus as claimed in claim1, wherein the at least one elongate stator assembly is decoupled fromthe first moveable member in at least one degree of freedom, and/or theat least one armature assembly is decoupled from the second moveablemember in at least one degree of freedom.
 19. A positioning apparatus asclaimed in claim 1, wherein the at least one elongate stator assembly isdecoupled from the first moveable member in at least one degree offreedom, and the at least one armature assembly is decoupled from thesecond moveable member in at least one degree of freedom.